The 29-year-old Kim drained a 10-foot birdie putt on the last hole for a final round two-under-par 68 and a winning total of nine-under-par 271 in the event co-sanctioned by the Asian and Japan golf tours.
Japan's Toshinori Muto, who briefly joined Kim in the lead finished tied for second with his countryman Yuta Ikeda while Michio Matsumura was a further shot back in fourth.
Kim, who started the day with a two-stroke lead, previously won the Diamond Cup in 2010 and has been in great form this year, winning four times after a recent lean run.
"I didn't expect to win so many titles this year," he said.
"I haven't won in three years before this season started so I was hoping to win at least one tournament."
Muto, a six-time winner in Japan, said he threw away his chance of winning when he bogeyed the 14th hole after an errant tee shot.
"It was a very good chance for me to win today and I know that," he said. "When I saw that I was tied for the lead, I wanted to be aggressive but it cost me a bogey. Kim played well so he deserves the win."
(Reporting by Julian Linden; editing by Sudipto Ganguly)
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